The present invention relates to an impact tool in which a hammer casing installed with an impact mechanism is mounted on a front section of a housing body housing a motor.
For example, Japanese Patent Publication No. 2003-145439A discloses an impact driver in which an internal gear casing and a hammer casing are mounted on a front section of a housing body which houses a motor. In the hammer casing, an impact mechanism is installed. This impact mechanism comprises: a spindle operable to rotate to transmit a power from an output axis of the motor by way of a planetary reduction gear mechanism; a hammer coupled to an outer periphery of the spindle so as to be movable in an axial direction of the spindle and so as to rotate together with the spindle; and an anvil disposed in a front side of the hammer so as to project toward a front side of the hammer casing. The hammer is urged toward the anvil by a coiled spring so that an engaging claw provided on a front face of the hammer is engaged with an arm provided on a rear end of the anvil.
With this configuration, when the motor is driven to rotate the spindle, the anvil is accordingly rotated by way of the hammer, so that a screwing operation can be performed by a bit attached on the anvil. If an excessive load is imparted on the anvil at a final stage of the screwing operation, the hammer is retracted against the urging force of the coiled spring and disengaged from the anvil. The disengaged hammer is then rotated together with the spindle and proceeded toward the anvil with the aid of the urging force of the coiled spring to again engage with the anvil. The disengagement and re-engagement are repeated to provide intermittent impacts on the anvil, thereby additional screwing forces are applied to finalize the screwing operation.
On the other hand, the internal gear casing is fixed on the housing body by screwing. Male screw portions provided on an outer periphery of a front end of the internal gear casing is screwed into female screw portions provided on an inner periphery of a rear end of the hammer casing, so that the internal gear casing and the hammer casing are coupled to each other. The undesired movement of the hammer casing relative to the internal gear casing in a circumferential direction thereof is prevented by fixing a lack on a lower face of the hammer casing in the housing body with screws in order to cause the lack to mesh with dimples provided on the outer periphery of the hammer casing.
In the above impact driver, since the fixation of the internal gear casing with respect to the housing body, and the fixation of the hammer casing with respect to the housing body are separately performed, and since the undesired rotation of the hammer casing relative to the housing body is prevented with different parts. Therefore, the number of parts will be naturally increased, and the assembling work well be troublesome. Further, there is a probability that the parts are fallen out when the impact driver is disassembled for the maintenance purpose.
Since the internal gear casing is covered with the hammer casing after the internal gear casing is fixed on the housing body with screws, it is difficult to downsize the hammer casing. Accordingly, the operability and the workability are not so good at a narrow space, for example.